Abstract: A laminar flow cooling system employs a laminar flow nozzle comprising a pair of plate members defining slit through which cooling water flows to form a cooling water screen. One of the plate members of the laminar nozzle is deformable at least in a direction perpendicular to the cooling water flow direction to adjust the path area in the nozzle. At least one of the plate member is preferably responsive to the cooling water pressure to cause variation of the path area for adjusting the cooling water path area.

Abstract: A laminar flow cooling system employs a laminar flow nozzle comprising a pair of plate members defining slit through which cooling water flows to form a cooling water screen. One of the plate members of the laminar nozzle is deformable at least in a direction perpendicular to the cooling water flow direction to adjust the path area in the nozzle. At least one of the plate member is preferably responsive to the cooling water pressure to cause variation of the path area for adjusting the cooling water path area.

Abstract: A system for online-detection of a transformation value and/or a flatness of a steel or a magnetic material includes: an exciting coil disposed at either side of the plate-shaped steel or magnetic material and capable of generating alternating magnetic fluxes by alternating current excitations; two or more detecting coils arranged on the same side as the exciting coil and at positions different in distance from the exciting coil, and mutually induced with the exciting coil; and an arithmetic unit for obtaining the transformation value and the flatness of the steel or the magnetic material or either one of those from a difference between detection signals generated from a difference in values of interlock magnetic fluxes in the respective detecting coils. The transformation value and the flatness of the steel or the flatness of the magnetic material can be detected with high accuracy.

Abstract: In controlling the cooling of a hot-rolled steel sheet after hot rolling, a target value of a transformation rate required for obtaining mechanical properties ultimately expected from the hot-rolled steel plate is previously determined, a rate of gamma to alpha transformed fraction of the hot-rolled steel sheet in a section in which the cooling is controlled is detected by transformed fraction detectors, and an elapsed time from the start of cooling is measured to determine the transformation rate of the steel sheet in a cooling stage, the conditions of cooling being controlled so that the transformation rate can coincide with the target value.

Abstract: The present invention aims to obtain C-Si-Mn-Cr system of hot-rolled dual phase structured steel sheets having a low yield ratio YR of not greater than 65%, an excellent strength-elongation balance M, a low variation in quality and an excellent cold formability through stepwise cooling regulation in the course of cooling from the final rolling to coiling.The present invention is a method for producing hot-rolled steel sheets having a low yield ratio and a high tensile strength due to dual phase structure by effecting the final rolling of a hot-rolled steel sheet containing 0.02-0.2% of C, 0.05-2.0% of Si, 0.5-2.0% of Mn and 0.3-1.5% of Cr as the essential components at a temperature of finishing the final rolling of 780.degree. C., rapidly cooling the thus treated steel sheet at a cooling rate of more than 40.degree. C./S to the temperature range wherein the transformation of .gamma. into .alpha.